A method for etching a dielectric layer is provided. A patterned mask with mask features is formed over a dielectric layer. The mask has isolated areas and dense areas of the mask features. The mask is trimmed by a plurality of cycles, where each cycle includes depositing a deposition layer, and selectively etching the deposition layer and the patterned mask. The selective etching selectively trims the isolated areas of the mask with respect to the dense areas of the mask. The dielectric layer is etched using the thus trimmed mask. The mask is removed.